this sure does sound like alot of fun you have gotten yourself into. wish i was there. i have the d44 front end on mine and have not torn it up yet. really dont know what all you plan on doing with this truck but mine has been through some pretty good mud and has held up.
beware of lincoln lockers. welded gears are a cheap way to get it accomplished and is really not the right way to do it. you mentioned that money is tight for the 400, which i think is a great choice for the truck, but you might save your money for a good detroit locker or the like.
for gear ratio, count the number of teeth on the ring gear. count the number of teeth on the pinion gear. divide the pinion count into the ring count. i.e. 42 teeth on the ring gear and 12 teeth on the pinion gear (42/12) would give you a 3.50 ratio.